Various processes are used in finishing textiles. Commercial drying operations include shrink drying and print drying. In shrink drying, a wet textile product is dried in order to shrink the textile and reduce the potential for the finished product to shrink further when subsequently laundered by the end user. Such textiles are used, for example, to make underwear and/or other knitted cotton goods. In print drying, the solvent from freshly applied dye is evaporated.
Textile drying is frequently performed in a heated air convection dryer. Typically, such dryers include at least one endless conveyor belt comprised of an air-permeable industrial fabric. The wet textile product is continuously deposited on the fabric at the dryer inlet and carried through the dryer. Drying is accomplished by blowing high temperature air at a very high flow rate through the textile product and, accordingly, the transporting fabric. High air flow rate is important in order to heat the wet textile material and drive off the liquid as quickly as possible. A textile shrink dryer is disclosed in U.S. Pat. No. 5,274,892.
To enable the passage of high air flow rates, textile dryer fabrics generally have a large fraction of open area. Consequently, it is desirable to minimize the yarn density and size used in the fabric's construction to result in a high permeability. A conventional method of measuring permeability is set forth in U.S. Pat. No. 4,290,209.
The fabric must also be suitably strong and abrasion resistant to withstand the stress of constant motion in the textile dryer. Additionally, the fabric must be hydrolyrically and thermally stable to resist degradation from exposure to the high temperature, moist environment that is prevalent during textile drying.
The use of certain types of synthetic yarns for weaving textile dryer fabrics is well known in the art. Conventional preferred materials include wholly aromatic polyamides such as poly-(m-phenyleneisophthalamide) and poly-(p-phenyleneterephthalamide), available commercially under the tradenames Kevlar.RTM. and Nomex.RTM., respectively. These materials are preferred since they are thermally and hydrolyrically stable and are readily formed into relatively strong multifilament yarns.
An example of a textile dryer fabric is the model T457 fabric available from Asten Specialty Fabrics, Walterboro, S.C. That fabric is flat woven in a plain leno weave with 12 warp yarns per inch by 7 filling yarns per inch. The warp yarns are 1200 denier/2 ply multifilament Nomex yarns and the filling yarns are 4 ply RFL coated glass with two wraps of 200 denier Nomex multifilament yarns. The warp and filling yarns are woven to produce a relatively open fabric having a hole size of approximately 0.13 inches by 0.12 inches and a resultant finished permeability of 1200 cfm. In order to provide sufficient stiffness to the fabric, the fabric is finished with a resin treatment.
The expected life of a textile dryer fabric is at least 1 to 11/2 years during which the fabric is subjected to the extremes of the textile drying process. With respect to print drying, the fabric is subjected to high volume flow of hot air on the order of 350.degree. F. to 390.degree. F. for normal running conditions. With respect to shrink drying, the textile drying environment is characterized by high volume air flow having a temperature of up to about 325.degree. F. with very high humidity due to the amount of moisture removal from the textile during the shrink drying process.
As compared with other industrial uses, such as papermaking, textile dryers run at relatively low speeds and textile fabrics are subject to relatively low tension. For example, print dryers run at a rate of approximately 300 feet per minute with fabric tension of approximately 4-6 pounds per linear inch. Speeds of shrink dryers typically do not exceed 150 feet per minute with conveyor fabric tension of approximately 3-5 pounds per linear inch. Comparatively, paper drying apparatus typically runs at speeds of 1000-4000 feet per minute with fabric tension ranging from about 6-15 pounds per linear inch.
In conventional paper drying apparatus, a papermakers dryer fabric conveys a paper product in contact with heated cylinders in comparison to the high volume hot air typified by textile drying apparatus. In the papermaking art there are special processes known as thru drying which employ thru-dryer fabrics having high open area to aid in the formation of a pillowing effect on the paper product. For example, see U.S. Pat. No. 5,114,777. However, the high volume flow hot air process employed in textile drying is quite different than the thru drying environment of the papermaking apparatus. In view of the different environment and processes involved, industrial fabrics for textile drying, conventional paper drying, and paper thru drying are in general markedly different. Fabrics designed for one such application cannot be viably substituted in another of such applications.
Other types of textile drying apparatus exist which operate at lower temperatures than the 350.degree. F.-390.degree. F. textile drying environment for the textile dryers of the present invention and at significantly less humidity conditions than the shrink dryers of the present invention. In such lower temperature, dry applications, fabrics made of polyester monofilament yarns having a permeability of approximately 700 CFM have proved satisfactory. Unsuccessful attempts have been made to produce a polyester monofilament fabric for a shrink drying application.
Although conventional textile fabrics made of multifilament yarns have provided acceptable performance to the textile drying industry, applicant has recognized that improvements can be made. During the use of conventional textile dryer fabrics, the resin begins to wear off. This can increase the fabric's susceptibility to contamination. For example, with respect to print drying, as the resin of the textile fabric wears off, the multifilament yarns become more susceptible to collecting excess dye which transfers from the textile being dried. Contaminants and the resin can also cause stiff protrusions from the multifilament yarns which have a tendency to "pick" threads out of the textile product causing product damage. Furthermore, as the resin wears and is exposed to continuous heat, the overall fabric loses its stiffness and the yarns become brittle. Accordingly, the fabric becomes susceptible to creasing, wrinkling and abrasive wear.
Applicant recognized that the use of monofilament yarns could solve some of the problems associated with the wear factors associated with conventional textile dryer fabrics. For example, Applicant recognizes that monofilament yarns are inherently stiffer than multifilament yarns of the same denier and that this advantage could be used to potentially eliminate the need for resin treatment. Applicant also recognized that monofilament yarns are inherently smoother than multifilament yarns so that the use of monofilament yarns would not be as susceptible to picking up contaminants or "picking" threads out of textile product being dried.